Every now and again I make an effort to get through a little bit more of passerine bird diversity (see the list of articles below for previous efforts). This is such an enormous and vastly diverse clade, alas, that I’ll probably never manage it – unless, that is, that I blog about passerines and not much else for a year or so. What I’ve decided to do here is compile various of the photos I’ve recently taken of assorted passerine species and use them as an excuse to talk about passerine diversity and phylogeny [UPDATE: as you'll see, the article ended up becoming a bit more restrictive in scope...]. It’s a bit like trying to walk across a river in flood – you plant your staff in the riverbed and that section of riverbed, and that section alone, is the bit you get to write about. Whether that analogy makes any sort of sense or not, off we go...

Oh, what I am getting at with the “view from the peripheries” thing referred to in the title? This is a reference to my pet idea that western Europe – and the British Isles in particular (my home base) – are very much a far-flung, peripheral region when it comes to the evolution and history of groups like the passerines. Think about it: phylogeny shows us that most of the key divergences in passerines happened in Australasia, tropical Asia and Africa (e.g., Beresford et al. 2005, Fuchs et al. 2006). Europe – strongly affected by a recent history of glaciations – has only been colonised by far-flying migrants or by reasonably hardy, cool-climate specialists that moved here after originating further to the south or east. So, in a biogeographical and evolutionary sense, I write to you ‘from the fringes’; from a far-flung, Atlantic outpost that should be considered last whenever we discuss evolutionary history, not first as it typically is due to historical, anthropocentric bias.

This is a Common chiffchaff Phylloscopus collybita, a familiar, ubiquitous leaf warbler here in western Europe. I photographed this particular bird in Carmarthenshire, Wales, and you wouldn't believe the time and effort it took to get the image you see here (or, if you photograph birds yourself, maybe you would). Leaf warblers (the Phylloscopus species) are slim-billed insectivores, greenish-greyish above and whitish below. There have been occasional efforts to recognise new genera or ‘subgenera’ within Phylloscopus (namely Cryptigata, Seicercus and Acanthopneuste), the result being radical non-monophyly and more confusion than ever (Olsson et al. 2005).

Where do leaf warblers and kin belong in passerine phylogeny? They’re uncontroversially part of Sylvioidea, the major passeridan passerine clade that includes all Old World warblers as well as bulbuls, cisticolas, white-eyes, Old World babblers and numerous similar birds (Passerida = the clade that contains all sparrow-like, finch-like and warbler-like passerines). However, the idea that all Old World warblers should be classified together is not supported by molecular data. Convention would have it that leaf warblers belong to Sylviidae - the 'Old World warbler family' - but the fact that recent phylogenies find Sylvia warblers to be closer to Old World babblers, white-eyes, cisticolids and so on means that this is very wrong, since leaf warblers and kin are clearly not part of this clade (Beresford et al. 2005, Alström et al. 2006, Johansson et al. 2008). The best way to classify leaf warblers may, actually, be to give them their own ‘family’ – Phylloscopidae – that’s close to (but distinct from) Cettidae and related sylvioid clades (Alström et al. 2006, Johansson et al. 2008).

As surprising and radical as it might seem, other Old World warblers conventionally discussed alongside leaf warblers – namely, the Locustella, Acrocephalus and Hippolais warblers – aren’t part of the same group as Phylloscopidae, Cettidae and so on, but are instead part of that bulbul-babbler-cisticola clade (Alström et al. 2006, Johansson et al. 2008). Remember this when you open your field guide and see all those warbler-type birds arranged together on the same few pages: they actually belong to at least three distinct evolutionary radiations, are well separated in the sylvioid tree (except for Acrocephalus and Hippolias, which really are close relatives), and are surrounded in the phylogeny by tropical African and Asian taxa. [Adjacent photos by Vogelartinfo, Dirk-Jan Kraan, Andreas Trepte and Lasse Olsson.]

Chiffchaffs are famously similar to Willow warblers Ph. trochilus in appearance (but not in voice: the name 'chiffchaff' is semi-onomatopoeic). Some studies indicate that both are essentially sister-taxa, only the Plain leaf warbler Ph. neglectus of central Asia being ‘intermediate’ between the two (it seems to be closer to chiffchaffs than to the Willow warbler) (Badyaev & Leaf 1997, Helbig et al. 1996, Olsson et al. 2005). Of course, this picture becomes more complex if various of the traditional chiffchaff ‘subspecies’ are recognised as worthy of species rank, like the Iberian chiffchaff Ph. brehmii, Canary Islands chiffchaff Ph. canariensis, Eastern or Mountain chiffchaff Ph. sindianus and Siberian chiffchaff Ph. tristis. Yes, as is so often the case, ‘the Chiffchaff’ is a polytypic entity that encompasses substantial diversity, with some of its ‘subspecies’ being 'more distinct' than many universally recognised ‘species’ in other bird groups, and easily differentiable based on morphology, acoustics, hybridisation patterns and DNA (Helbig et al. 1996). [Chiffchaff below by Andreas Trepte; Willow warbler by Rob Bendall/Highfields.]

Regardless, Common chiffchaff and Willow warblers are known to hybridise in places (the hybrids have a ‘hybrid song’, featuring song phrases of both parent species) (da Prato 1993). Despite the stereotype about them being similar except for voice, they’re actually not that difficult to distinguish if you know what to look for: Willow warblers are larger with a shallower, less domed head, slightly longer primary projection, and they’re 'brighter', with light brownish/pinkish legs (as opposed to darker grey legs) and have a more washed-out look overall. The habitat preferences of the two are also slightly different. Chiffchaffs are generally restricted to areas where there are tall deciduous trees whereas Willow warblers are more associated with woodland edges, scrubland and clearings. However, the two must be pretty similar in ecological terms since some studies indicate that, when one of the two species occupies a habitat, the other one is discouraged from doing so (Saether 1983). Willow warblers are somewhat larger so tend to have the advantage if they occupy a habitat first (Saether 1983).

This article was written as a quick summary, the original plan being that I was going to use it to showcase some of my Welsh chiffchaff photos. It goes without saying that a huge amount of research has been done on leaf warbler hybridisation, distribution, ecology, foraging behaviour, systematics and phylogeny, and you can all help to make this article (and its comments) fantastically complete by saying smart and brilliant things in the comments below. So, over to you... (whackaloons and creationists: let’s see if you can find things to say when Mesozoic dinosaurs and primates aren’t involved).

More Old World passerines as seen "from the peripheries" later. Next: TITS (again). For previous Tet Zoo articles on passerines, see...

Saether, B. 1963. Mechanisms of interspecific spacing out in a territorial system of the Chiffchaff, Phylloscopus collybita, and the Willow warbler, Phylloscopus trochilus. Ornis Scan 14, 154-160.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Darren Naish

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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